One type of pneumatic fender is a vertical pneumatic fender comprising a bob connected to a lower hemispherical portion and containing a predetermined amount of ballast water in an internal space (see, for example, Japanese Unexamined Patent Application Publication No. H11-117261A). The stability of the position of the vertical pneumatic fender is maintained by balancing the vertical downward force of the bob and the ballast water in the internal space and the buoyancy of the fender.
Pneumatic fenders normally have a cylindrical-shaped body portion with bowl-shaped mirror-sections provided at both ends, and the body portion has a layered configuration with a plurality of reinforcing layers between inner rubber layer and outer rubber layer. The reinforcing layers are cord layers formed by arranging a plurality of cords in parallel, and the cords are disposed at a predetermined cord angle with respect to the axial direction of the cylinder. The cords of adjacently layered upper and lower reinforcing layers are disposed so as to intersect one another. Conventionally, the cord angle is set to roughly an angle of repose (54° to 55°) in a neutral state, with the result that there is relatively little change in the size (i.e., length and outer diameter) of the body portion even when the interior of the pneumatic fender is inflated to specified internal pressure.
When a moored vessel suddenly rocks due to a tsunami or the like, causing the fender to be excessively compressed by greater than expected berthing impact energy, a safety valve opens, venting air within the internal space to the outside. If the safety valve remains open during this process, an excessive amount of air may be vented to the outside, causing the fender to sink due to insufficient buoyancy in the worst-case scenario. Conversely, if the fender is compressed too rapidly compared to the speed at which air is vented to the outside, the internal pressure of the fender will suddenly increase, potentially causing the fender to burst in the worst-case scenario. There is therefore a demand for a vertical pneumatic fender that is capable of responding to sudden increases in berthing impact energy.